Knowledge Bank

In the presence of very intense light (e.g. ruby laser light), molecules can absorb two photons simultaneously, even though neither photon alone could be absorbed by the molecule. The selection rule for this process is the reverse of the one photon rule: for two photon transitions, $g\to g$ is allowed and $g\to u$ is forbidden. Thus two-photon absorption spectra reveal the presence of molecular excited states which are hidden to ordinary spectroscopy. A two-photon absorption spectrometer has been built which allows the measurement of all three of the molecular parameters governing the two-photon absorption of randomly oriented molecules. This is accomplished by the use of both linearly and circularly polarized light in the spectrometer, as described in a previous theoretical $paper.1$ The design of the spectrometer allows particularly accurate measurement of the ratios of the three absorption parameters, since these ratios enter into the application of two exact symmetry $rules1$ which permit one to determine the symmetry of the upper state in allowed two-photon transitions.